TODO:
I still think we can interleave layers to get better progressive loading. But we need FC, FD, FE, and FF commands for "rerun 4/3/2/1 tiles ago."
Make a super simple one corresponding to quality 3 or so, controlled by constants?
Generally improve code quality.


Done:
Actually support terminating early for "progressive" loads
Support an alpha channel.
Improve the PNG interface.
Interleave layers instead of having them consecutively. (Bad idea, breaks RLE. Skipping)
Global parameters to control encoder settings:
- Quantization level: quant 0
- Quantization level: quants 1-63
  - Constant term
  - Linear term
- Number of quants to keep, per tile.
- Whether to use wide or packed representation for the thumbnail block
Maybe have a second instance of the global parameters block for the alpha channel.